The invention relates generally to apparatus for detecting the presence of phase encoded data and, more particularly, to apparatus for detecting the presence of phase encoded data in a data storage subsystem employing a magnetic recording medium.
In contemporary data processing systems data is stored on magnetic tape or magnetic disks for retrieval and use at a later time. It is important that large quantities of data be stored as compactly as possible to minimize the number of reels of tape or number of disks used with the data processing system. One of the techniques used to increase the quantity of data which can be stored in a given space is known as phase encoding. In phase encoding data bits are represented by specific changes in voltage level. For example, a binary zero may be represented consistently by an increasing signal voltage, and a binary one may be represented consistently by a decreasing signal voltage. When a series of binary ones or a series of binary zeros is recorded, it is necessary to include a "phase bit" between the binary ones or between the binary zeros. The phase bit may be used to synchronize the data with a variable frequency oscillator in a data recovery portion of the controller being used to direct the operations of the particular magnetic storage device. This synchronization enables the data recovery portion of the controller to read the data only at those times when signal voltage level changes represent real data, so that noise voltages which may occur at other times will be screened out and not be registered as real data.
Data is stored in a magnetic tape subsystem in "blocks" with each block of data comprising a preamble portion, a data portion, and a postamble portion. The preamble is used to synchronize the variable frequency oscillator in the data recovery portion of the magnetic tape subsystem controller. The oscillator in turn is used to enable the tape subsystem controller to sample or read each of the data pulses near the middle of the pulse so that noise and other interference will not introduce error signals into the data processing system. According to the embodiment of the invention described herein, the magnetic tape subsystem stores and retrieves data in which the preamble and postamble data portions each comprise a series of 80 pulses. The 80 pulses of the preamble are utilized to adjust the oscillator so that it will be in exact synchronization with the data pulses being detected. The data envelope detector of the present invention is used to sense the presence of data as it is written onto or read from the magnetic tape, and it generates an output signal which turns on a certain time after the preamble portion has been detected and turns off after an equivalent amount of time has expired after the end of the postamble has been detected. The output signal generated by the data envelope detector is used by other circuitry for performing various control functions within the tape subsystem controller. Reference may be had to U.S. Pat. No. 3,832,684, assigned to the assignee of the present invention, for an example of such control circuitry.
A known prior art data envelope detector is described in U.S. Pat. No. 3,810,233, assigned to the assignee of the present invention, wherein an output signal is generated in every case at a fixed time (i.e., after a fixed number of data pulses) after detection of the preamble, and wherein the output signal returns to zero at an equivalent amount of time after detection of the end of the postamble. While the operation of the prior art envelope detector is satisfactory under nearly all circumstances, it is desirable to have an envelope detector in which the time delay between detection of the preamble and generation of an output signal is variable depending upon the nature of the data transfer operation.